Intrinsic Deformation Behavior Research Articles

Compositionally graded specimen made by laser additive manufacturing as a high-throughput method to study radiation damages and irradiation-assisted stress corrosion cracking

This study demonstrates the feasibility of using compositionally gradient specimens, fabricated by laser additive manufacturing (AM) and post-AM thermo-mechanical treatment, to accelerate alloy synthesis, radiation experiment, and the assessment of irradiation properties in light water reactor environments. The effects of minor Hafnium (Hf) doping in austenitic 316L stainless steel (SS) was selected as the topic of interest. By comparing to the data in literature, we confirmed that the compositionally graded specimen produces the same trend of void swelling, dislocation loops, radiation-induced segregation (RIS), radiation hardening as the wrought specimen produced by cast/forging process. Hf suppressed most radiation damages through strong interaction with point defects. The work also demonstrates the use of compositionally gradient specimens to study the irradiation-assisted stress corrosion cracking (IASCC) susceptibility of Hf-modified SS. While the suppression of radiation hardening and RIS are consistent with the IASCC mitigation by Hf, we emphasize Hf can alter the intrinsic deformation behavior of 316L SS, which reduces grain-boundary strain localization. The advantages and challenges of using compositionally gradient design for high-throughput nuclear alloy development and qualification are also discussed.

Penta-Twin Destruction by Coordinated Twin Boundary Deformation.

Penta-twinned nanomaterials often exhibit unique mechanical properties. However, the intrinsic deformation behavior of penta-twins remains largely unclear, especially under the condition of high shear stress. In this study, we show that the deformation of penta-twins often subject to a structural destruction via dislocation-mediated coordinated twin boundary (TB) deformation, resulting in a reconstructed pentagon-shaped core. This reconstructed core region is mainly induced by the coordinated TB migration along different directions (for the nucleation and growth) and accelerated by the TB sliding (for the growth). The destructed penta-twin core can effectively accommodate the intrinsic disclination of the penta-twin, which further collapses beyond a critical size, as predicted by an energy-based criterion. These intrinsic deformation behaviors of penta-twins would enable the possibility of controlling the morphology of penta-twinned nanomaterials with unique properties.

Gradient cell-structured high-entropy alloy with exceptional strength and ductility.

Similar to conventional materials, most multicomponent high-entropy alloys (HEAs) lose ductility as they gain strength. In this study, we controllably introduced gradient nanoscaled dislocation cell structures in a stable single-phase HEA with face-centered cubic structure, thus resulting in enhanced strength without apparent loss of ductility. Upon application of strain, the sample-level structural gradient induces progressive formation of a high density of tiny stacking faults (SFs) and twins, nucleating from abundant low-angle dislocation cells. Furthermore, the SF-induced plasticity and the resultant refined structures, coupled with intensively accumulated dislocations, contribute to plasticity, increased strength, and work hardening. These findings offer a promising paradigm for tailoring properties with gradient dislocation cells at the nanoscale and advance our fundamental understanding of the intrinsic deformation behavior of HEAs.

Minimal effect of stacking number on intrinsic cleavage and shear behavior of Tin+1AlCn and Tan+1AlCn MAX phases

MAX phases are layered carbides or nitrides with the general formula Mn+1AXn, which exhibit a unique combination of ceramic- and metal-like properties. The effect of stacking a number (determined by n) remains to be elucidated and a priori is not clear whether, for a given chemistry, n significantly changes the intrinsic deformation behavior of these systems. In this work, we have studied the intrinsic deformation behavior of Tin+1AlCn and Tan+1AlCn (n = 1 … 5) using DFT-based calculations. Surprisingly, the results suggest that the stacking number tends to have a minimal effect on the intrinsic mechanical behavior of the systems studied.

Revisiting intrinsic brittleness and deformation behavior of B2 NiAl intermetallic compound: A first-principles study

For NiAl intermetallic compound with B2 structure, there is still no calculation combining models of single and multiple layers while using the same basic set. Furthermore, some recently proposed criteria for brittleness and toughness have not been used to analyze its deformation behavior. Thus, first-principles calculation was applied to comprehensively study the elastic properties, ideal strength, generalized stacking fault energy and surface energy of B2-NiAl intermetallic compound. The results suggest that calculations based on the current basic set give more accurate lattice parameters and elastic moduli. The Pugh criterion and Cauchy pressure cannot be used to interpret the intrinsic brittleness of NiAl. In comparison, the ductility parameter based on the strain energy under elastic instability and ZCT and Rice criteria based on generalized stacking fault energy and surface energy successfully identify the intrinsic brittleness of the NiAl intermetallic compound. The reason why [111] slip always occurs in the deformation along [100] direction was clarified by examining the critical value for brittle-ductile transition. The results of density of state indicate shear deformation has less impact on structural stability, and the change of charge density difference implies that <001> shear induces more intensive redistribution of charge density, which is well correlated to the brittleness and deformation behavior of NiAl intermetallic compound.

Fracture behaviors under pure shear loading in bulk metallic glasses.

Pure shear fracture test, as a special mechanical means, had been carried out extensively to obtain the critical information for traditional metallic crystalline materials and rocks, such as the intrinsic deformation behavior and fracture mechanism. However, for bulk metallic glasses (BMGs), the pure shear fracture behaviors have not been investigated systematically due to the lack of a suitable test method. Here, we specially introduce a unique antisymmetrical four-point bend shear test method to realize a uniform pure shear stress field and study the pure shear fracture behaviors of two kinds of BMGs, Zr-based and La-based BMGs. All kinds of fracture behaviors, the pure shear fracture strength, fracture angle and fracture surface morphology, are systematically analyzed and compared with those of the conventional compressive and tensile fracture. Our results indicate that both the Zr-based and La-based BMGs follow the same fracture mechanism under pure shear loading, which is significantly different from the situation of some previous research results. Our results might offer new enlightenment on the intrinsic deformation and fracture mechanism of BMGs and other amorphous materials.

Electron Beam Induced Artifacts During in situ TEM Deformation of Nanostructured Metals

A critical assumption underlying in situ transmission electron microscopy studies is that the electron beam (e-beam) exposure does not fundamentally alter the intrinsic deformation behavior of the materials being probed. Here, we show that e-beam exposure causes increased dislocation activation and marked stress relaxation in aluminum and gold films spanning a range of thicknesses (80–400 nanometers) and grain sizes (50–220 nanometers). Furthermore, the e-beam induces anomalous sample necking, which unusually depends more on the e-beam diameter than intensity. Notably, the stress relaxation in both aluminum and gold occurs at beam energies well below their damage thresholds. More remarkably, the stress relaxation and/or sample necking is significantly more pronounced at lower accelerating voltages (120 kV versus 200 kV) in both the metals. These observations in aluminum and gold, two metals with highly dissimilar atomic weights and properties, indicate that e-beam exposure can cause anomalous behavior in a broad spectrum of nanostructured materials, and simultaneously suggest a strategy to minimize such artifacts.

Intrinsic or extrinsic size-dependent deformation behavior of a Zr-based bulk metallic glass in supercooled liquid state?

The size-dependent deformation behavior of Zr65Cu17.5Ni10Al7.5 bulk metallic glass in supercooled liquid state was investigated through finite element simulations combined with the free volume constitutive relation, based on both uniaxial compression and tension. Taking into consideration the extrinsic effect (i.e. interfacial friction) in compression, the simulated results revealed a relatively more homogeneous flow and free volume distribution in the Zr-based bulk metallic glass with larger sample size. While under uniaxial tension without any interfacial friction but with the same temperature, stain and strain rate as what were used in compressive experiments, the simulated effective stress, strain and free volume concentration are almost identical at all sample sizes, denoting an intrinsic size-independent deformation behavior.

Fatigue Life Predictions for Glassy Polymers: A Constitutive Approach

Long-term failure under constant or cyclic load is governed by the same process as short-term failure at constant rate of deformation. Failure proves to originate from the polymer’s intrinsic deformation behavior, more particularly the true strain softening after yield, which inherently leads to the initiation of localized deformation zones. In a previous study we developed, and validated, a 3D constitutive model that is capable to predict the occurrence of these plastic instabilities, yielding quantitative predictions of the lifetime of polycarbonate under constant load.(1) Here we demonstrate that the same approach is also applicable to predict the life span of polycarbonate under cyclic loading conditions, over a large range of molecular weights and thermal histories, with a single parameter set only. The model incorporates the influence of physical aging, accelerated by the applied cyclic stress. For low cycle fatigue, at large stress amplitudes, where failure is thermally dominated, it is shown that ...

Quantitative Prediction of Long-Term Failure of Polycarbonate

Time-to-failure of polymers, and the actual failure mode, are influenced by stress, temperature, processing history, and molecular weight. We show that long-term ductile failure under constant load is governed by the same process as short term ductile failure at constant rate of deformation. Failure proves to originate from the polymer's intrinsic deformation behavior, more particularly the true strain softening after yield, which inherently leads to the initiation of localized deformation zones. In a previous study, we developed a constitutive model that includes physical aging and is capable of numerically predicting plastic instabilities. Using this model the ductile failure of polycarbonates with different thermal histories, subjected to constant loads, is accurately predicted also for different loading geometries. Even the endurance limit, observed for quenched materials, is predicted and it is shown that it originates from the structural evolution due to physical aging that occurs during loading. For low molecular weight materials this same process causes a ductile-to-brittle transition. A quantitative prediction thereof is, however, outside the scope of this paper and requires a more detailed study.

Intrinsic Deformation Behavior of Semicrystalline Polymers

The influence of crystallinity and lamellar thickness on the intrinsic deformation behavior of a number of semicrystalline polymers is studied: a poly(ethylene terephthalate) and two different molecular weight grades of polyethylene and polypropylene. The crystallinity and lamellar thickness are altered by varying the rate of crystallization from the melt and by cold crystallization (annealing) at elevated temperatures above Tg but below the melting point. Crystallinity and lamellar thickness are determined by wide-angle X-ray diffraction and small-angle X-ray scattering measurements. Uniaxial compression tests are performed to obtain the large strain intrinsic deformation behavior, e.g., yield stress, strain softening, and strain hardening modulus. The yield stress is found to be proportional to lamellar thickness, whereas the strain hardening modulus is shown not to depend on crystallinity or lamellar thickness. Over the strain range experimentally covered, the strain hardening modulus appears to be well described by a simple neo-Hookean relation and appears to be related to the chain entanglement density. An affirmation for this relation arises from the observation that slowly melt crystallized samples exhibit a lower strain hardening, resulting from a lower chain entanglement density, which is expected to be caused by reeling in of the molecular chains in such a slow crystallization process. The similarity in the results observed on all polymers tested supports the conclusion that the crystalline phase does not contribute to strain hardening, which is primary controlled by the chain entanglement density.

The influence of hydrogen bonding on the preparation and mechanical properties of PS‐diblock copolymer blends

AbstractIn this article, the preparation of nanosized core‐shell particles to induce ductility in polystyrene (PS) is described. FTIR spectroscopy, solid‐state NMR spectroscopy, and DSC were used to examine the extent of miscibility of PS and poly(butylacrylate)‐b‐polyolefin diblock copolymers in a blend in which PS was chemically modified by copolymerization with 0.5–5 mol % of p‐(hexafluoro‐2‐hydroxy isopropyl) styrene (HFS). Hydrogen bonding between the hydroxyl‐groups and the carbonyl‐groups of polybutylacrylate enhanced the miscibility and lead to randomly distributed polyolefin particles surrounded by a homogeneous PBA/PS matrix. Morphological parameters such as the size of the dispersed phase or extent of interpenetration between the components are controllable simply by changing the amount of interacting groups in the blend. The mechanical properties of the prepared blends were also studied. The intrinsic deformation behavior was investigated by compression tests, whereas the microscopic mode of deformation was studied by time‐resolved small‐angle X‐ray scattering. It was shown that the macroscopic strain at break depends to a large extent on the diblock copolymer content and the degree of demixing between the rubber shell and PS matrix. Brittle behavior was observed for PS blends that contain more than 3 mol % HFS and show complete miscibility between the PS matrix and acrylate shell. For the blends showing partial miscibility, the compression tests demonstrated a pronounced decrease in strain softening with increasing diblock copolymer concentration. Furthermore, it was illustrated that dependent on the degree of demixing the microscopic deformation mode changes from crazing to cavitation induced shear yielding. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2137–2160, 2004

Cyclic stress response characteristics of an aluminum–magnesium–silicon alloy

In this paper, the cyclic stress response characteristics of aluminum alloy 6061 is presented. Test specimens of the alloy were cyclically deformed using fully-reversed tension–compression loading under total strain amplitude control. The alloy showed evidence of initial hardening followed by progressive softening to failure. The stress response, deformation characteristics and fracture behavior of the alloy is interactively discussed in light of the competing influences of intrinsic microstructure contributions and deformation behavior arising from a combination of mechanical and microstructural contributions.